Hi Masters,
I have confusion on the LY/LZ/UNT/UNB definition in STAAD.
Usually, LY/LZ is used for column design to consider the unbraced length, and UNT/UNB is used for beam design to consider unsupported length.
However, beam can still be treated as axis compression/tension member in frame when horizontal load imposed, which is the same with column, shall LY/LZ be defined for beam as well?
In addition, can you please advise how LTB length is defined for column in STAAD?.
LY/LZ are used to calculate the compression capacity of the member and UNT/UNB are used to calculate the flexural capacity for lateral torsional buckling. Both can be used for columns as well as for the beams.
Hi Modestas,
What you response is description given in STAAD tutorial, but think about how to define UNT/UNB for column member (what is top/bottom flange)? Is there any change after setting LY/LZ for beam member?
Thanks.
UNT is for specifying the value of unsupported length when the top flange is in compression. The term "Top" signifies the flange on the positive side of the local Y axis. UNB is for specifying the value of unsupported length when the bottom flange is in compression. You can turn on local axis of the members by clicking shift+O on the keyboard. So the UNB and UNT assignment depends on the local Y orientation of the member and it does not matter if this is a beam or a column.
Modestas said:LY/LZ are used to calculate the compression capacity of the member and UNT/UNB are used to calculate the flexural capacity
Hello Modestas, i'm sailing the same boat...
i was so confident until a junior colleague raised some intriguing questions which left me dumbstruck. ... so i decided to try to mark out the LY/LZ/UNL/EFT values for different configuration (that my colleague is working on) and have attached here. Can you kindly review if my marking is right, and please correct me with valuable suggestion if it's wrong.
PDF
(pls excuse the image/link of Risa, as i couldn't find a more relevant snap elsewhere)
Thanks in advance.
(by the way, we're looking at the Parameters for EC3)
It is not possible to tell if LY and LZ are assigned correctly, without knowing the local axis system of the member. In general, I would suggest you to check this Wiki which clearly explains when LY and LZ could have different values. How to correctly define unsupported lengths for steel design - RAM | STAAD | ADINA Wiki - RAM | STAAD | ADINA - Bentley Communities
Are you using Tapered members in the design? Please note that EFT parameter is required for tapered members only.UNL should be a distance between the bracing points which provide restraints from later-torsional buckling.